Since a few of the bits in this bloody Hardinge apron project could do with hardening after they've been made and since I haven't yet built a decent toolpost grinder. I thought I'd try an experiment to see how much distortion I might get when hardening the steel that I'm using to make the gears and shafts from.
I'd read, via Tubal Cain and others, that salt baths can be very handy things to heat steel up in. They should be less likely to over/under heat bits of the part you are heating than an open torch and you shouldn't get the scaling problems you would in an open atmosphere.
The steel I'm using is EN24 aka AISI 4340. This steel is hardened by austenitizing at 802 Centigrade and quenching in oil and (in my case) tempering at 200 centigrade.
It just so happens that common table salt melts at 801 centigrade.
Armed with this knowledge and the fact that SWMBO had over-ordered on salt when making a batch of play-dough for some kids lead to today's experiment.
I measured the trial part as well as I could and noted the dimensions on the drawing I had made it from. Then set up a crucible on some fire bricks and surrounded it with the rockwool that I've used in the past for heating things in. A pound or so of salt was poured into the dried out crucible and set to heat. While that was warming up I set up a paint kettle of new 15W-40 motor oil on a gas ring for the purposes of quenching to the desired temperature.
I was rather worried that the oil was approaching its flash point and only heated it to 170 centigrade instead of the 200 that I had wanted... must get some heavier oil.
The pictures are here:-
Having a pot fill of red hot liquid with nothing to stop it from tipping over is a little bit scary. I think that a properly built salt bath is definitely there in the future. I was very impressed by the complete lack of scale on the steel.
I don't yet know whether I achieved the hardness that I was aiming for. The part isn't file-hard, but is very hard. This is what I was expecting, since I was aiming at about 42HRC. I'll see if I can get it tested at work tomorrow to put my mind at rest.
It warmed the shed up as well :-)
I must put a heat spreader under the firebricks before I do the next one though. I ended up with the poor old workmate smouldering before I'd finished.
Mark Rand RTFM